Could you be more specific?  For example, do you want to create foliage to make your own plants or trees? or do you want to create a green texture to go on a land terrain?  Do you want to create procedural materials or create bitmaps to go on your objects.  What you want to achieve, and how you explain it, will GREATLY depend on the answer you receive and will help eliminate answers that aren't what you are after.

Jess trying to be helpful